Method and apparatus for joint processing and precoding mode selection based on limited feedback in mobile communication system

ABSTRACT

According to one embodiment, a method of a base station configured to select a precoding mode in a mobile communication system. When channel estimate information is received from at least one terminal, the channel estimate information is collected, so that user grouping is performed according to one precoding scheme from among multiple precoding schemes. Individual user scheduling is performed according to each precoding scheme, so that a service candidate group set is generated. The selected precoding scheme is maximized according to a transmission capacity of a service candidate group set for each precoding scheme. Data is then transmitted to a determined candidate group set using the determined precoding scheme.

CROSS REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY

The present application is related to and claims the benefit under 35U.S.C. §119(a) of a Korean patent application filed in the KoreanIntellectual Property Office on Feb. 24, 2011 and assigned Serial No.10-2011-0016340, the entire disclosure of which is hereby incorporatedby reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to mobile communication systems,and more particularly, to a method and an apparatus for joint processingand precoding mode selection based on limited feedback in a mobilecommunication system.

BACKGROUND OF THE INVENTION

Various types of precoding schemes may be used depending on a spatialposition or time-varying conditions in a mobile communication system.Nevertheless, each precoding scheme may have different characteristics.That is, since each precoding scheme has a particular correlationbetween complexity of a processing procedure for transmission and afeedback weight generated by a terminal, each precoding scheme may havea performance difference depending on time-varying channel environmentsin a limited feedback environment.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, it is aprimary aspect of the present invention to provide a method and anapparatus for joint processing and precoding mode selection based onlimited feedback in a mobile communication system.

Another aspect of the present invention is to provide a method and anapparatus for obtaining a higher transmission capacity by selecting amore efficient transmission technique within a range of feedbacktechniques having a fixed number of precoding schemes in a mobilecommunication system.

Still another aspect of the present invention is to provide a method andan apparatus for selecting a precoding scheme having a maximum capacityfrom among multiple precoding schemes having differences in transmissioncapacity depending on a channel environment in a mobile communicationsystem.

In accordance with an aspect of the present invention, a method of abase station for selecting a precoding mode in a mobile communicationsystem includes, when receiving channel estimate information from atleast one terminal, collecting the channel estimate information toperform user grouping according to a precoding scheme from amongmultiple precoding schemes, performing individual user schedulingaccording to each precoding scheme to generate a service candidate groupset, determining a precoding scheme that maximizes a transmissioncapacity of a service candidate group set for each precoding scheme, andtransmitting data to a determined candidate group set using thedetermined precoding scheme.

In accordance with another aspect of the present invention, a method forselecting a precoding mode in a mobile communication system of aterminal includes performing channel estimation on a reference signalreceived from a base station to select a precoding scheme from amongmultiple precoding schemes, feeding back channel information regarding adetermined precoding scheme to the base station, and receiving data fromthe base station according to the determined precoding scheme.

In accordance with still another aspect of the present invention, a basestation configured to select a precoding mode in a mobile communicationsystem includes a user grouping unit configured to, when receivingchannel estimate information from at least one terminal, collecting thechannel estimate information to perform user grouping according to aprecoding scheme from among multiple precoding schemes, a schedulerconfigured to perform individual user scheduling according to eachprecoding scheme to generate a service candidate group set, anddetermine a precoding scheme that maximizes a transmission capacity of aservice candidate group set for each precoding scheme, and a transmitterconfigured to transmit data to a determined candidate group set usingthe determined precoding scheme.

In accordance with yet another aspect of the present invention, aterminal, configured to select a precoding mode in a mobilecommunication system includes a receiver configured to perform channelestimation on a reference signal received from a base station, andreceive data from the base station according to a determined precodingscheme from among multiple precoding schemes, a mode selector configuredto select a precoding scheme based on a channel estimate result, and atransmitter configured to feeding back channel information regarding adetermined precoding scheme to the base station.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, itmay be advantageous to set forth definitions of certain words andphrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning, and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates an example mobile communication system according toan embodiment of the present invention;

FIG. 2 illustrates an example message flow according to an embodiment ofthe present invention;

FIG. 3 illustrates an example process for operating a terminal accordingto an embodiment of the present invention;

FIG. 4 illustrating an example process for operating a base stationaccording to an embodiment of the present invention;

FIG. 5A illustrates an example performance analysis result of a systemaccording to an embodiment of the present invention; and

FIG. 5B illustrates an example performance analysis result of a systemaccording to an embodiment of the present invention.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components and structures.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 5B, discussed below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged wireless communication system.

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. Also, descriptions of well-known functions and constructionsare omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention are provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

Exemplary embodiments of the present invention provide a method and anapparatus for a cooperative process and precoding mode selection intransmission based on limited feedback in a mobile communication system.

More particularly, the present invention relates to a method and anapparatus for determining a precoding mode in order to efficientlychange a precoding scheme depending on a channel state under aCoordinated Multi-Point transmission Joint Processing/Transmission(CoMP-JPT) environment based on a limited feedback in a mobilecommunication system.

Generally, precoding schemes of a CoMP-JPT environment may be roughlyclassified into three types. First, multiple base stations may beregarded as one large super-base station (BS) and a channel operatingbetween one terminal and the super-base station is extended, so that aspatial diversity is obtained. The present invention defines this as aglobal precoding scheme. Second, a local precoding scheme of receivingdata from a plurality of base stations at the same time under anindependent channel environment may be enabled. Last, a Single FrequencyNetwork (SFN) precoding scheme that sums channels between multiple basestations and one terminal and uses the same may be enabled.

In the present invention, a terminal that receives a cooperative (orjoint) transmission service of multiple base stations determines aChannel Quality Indicator (CQI) and a Channel Direction Indicator (CDI)estimated depending on a channel change, determines a precoding schemehaving a highest CQI in feeding back relevant information in a limitedmanner, and performs CQI and CDI feedback on the determined precodingscheme.

A base station that receives feedback information classifies channelenvironment information of the terminal according to a particularprecoding scheme, and shares the information with base stations thatperform the cooperative transmission technique via a backhaul network.

The base stations that perform the cooperative transmission finallyselect whether to provide a service to terminals classified according tothe determined precoding scheme and provide the service.

In the case where a base station determines a precoding scheme via whicha service is received depending on a channel change and efficientlyselects terminals classified according to the determined precodingscheme, a link transmission capacity formed depending on a channelchange of a terminal may increase the efficiency of communication.

In the present invention, a base station shares channel information of aterminal via a backhaul network. With respect to the number K of allterminals, it is assumed that the number of transmission antennas of thebase station is N_(t), and a reception antenna of the terminal is asingle antenna. Generally, a reception signal of a terminal k servicedby a base station b may be expressed by Equation (1). In the presentinvention, the terminal may also be referred to as a user.

$\begin{matrix}{y_{b,k} = {{\sqrt{\rho \; h_{b,k}}w_{b,k}s_{b,k}} + {\underset{\underset{{Inter}\text{-}{user}\mspace{14mu} {interference}}{\uparrow}}{\sqrt{\rho \; h_{b,k}}\sum\limits_{{j \in S_{b}},{j \neq k}}}\; w_{b,j}s_{b,j}} + {\sqrt{\rho}{\underset{\underset{{Inter}\text{-}{cell}\mspace{14mu} {interference}}{\uparrow}}{\sum\limits_{{b^{\prime} \in B},{b^{\prime} \neq b}}}\; {h_{b^{\prime},k}{\sum\limits_{l \in S_{b}}\; {w_{b^{\prime},l}s_{b^{\prime},l}}}}}} + n_{b,k}}} & (1)\end{matrix}$

where h_(b,k) represents an 1×N₁ channel vector between a home basestation b and a terminal k, w_(b,k) represents an N_(t)×1 transmitweight vector, s_(b,k) represents a transmit symbol, h_(b,k) representsa 1×N_(t) interference channel vector between a neighbor base station b′and a terminal k, and w_(b′,1) and s_(b′,1) are an N₁×1 transmit weightvector of a neighbor base station b′ and a transmit symbol,respectively, S_(b) represents a set of terminals serviced by a basestation b, and B represents a set of all base stations.

A signal at a reception end has an Inter-user Interference (IUI) thatoccurs when a home cell services multiple terminals, and an Inter-cellInterference (ICI) that occurs when a neighbor cell services otherterminals.

Generally, in the case where a home base station services a terminalbased on a Zero-forcing Beamforming (ZFBF), an IUI with respect to arelevant terminal may be cancelled. In addition, under a CoMP-JPTenvironment, an ICI from a base station performing a cooperativetransmission service may be cancelled. However, when limited feedback isassumed, an ICI may not be completely canceled.

Under the CoMP-JPT environment, a reception signal of a terminal k maybe expressed using Equation (2) depending on a precoding scheme.

$\begin{matrix}{y_{b,k} = {{\sqrt{\rho \;}h_{b,k}^{({Mode})}w_{b,k}^{({Mode})}s_{b,k}} + {\underset{{Inter}\text{-}{user}\mspace{14mu} {{interference}{(x)}}}{\sqrt{\rho \;}h_{b,k}^{({Mode})}\sum\limits_{{j \in S_{b}},{J \neq k}}}\; w_{b,j}^{({Mode})}s_{b,j}} + {\sqrt{\rho}\underset{{Inter}\text{-}{cell}\mspace{14mu} {interference}}{\sum\limits_{{\hat{b} \in B},{\hat{b} \neq b},{\hat{b} \neq b^{\prime}},{\hat{b} \neq b^{''}}}\; {h_{\hat{b},k}{\sum\limits_{l \in S_{b^{\prime}}}\; w_{\hat{b},l}}}}s_{\hat{b},l}} + n_{b,k}}} & \; \\{h_{b,k}^{({Mode})} = \left\{ \begin{matrix}{{h_{b,k}^{({Global})}w_{b,k}^{({Global})}} = \left\lbrack \begin{matrix}h_{b,k} & h_{b^{\prime},k} & {\left. h_{b^{''},k} \right\rbrack\left\lbrack \begin{matrix}w_{b,k} & w_{b^{\prime},k} & \left. w_{b^{''},k} \right\rbrack^{T}\end{matrix} \right.}\end{matrix} \right.} \\{{{h_{b,k}^{({Local})}w_{b,k}^{({Local})}} = \left\lbrack \left( {\sum\limits_{\overset{\_}{b}}\; {h_{\overset{\_}{b},k}w_{\overset{\_}{b},k}}} \right) \right\rbrack},{\overset{\_}{b} = b},b^{\prime},b^{''}} \\{{{h_{b,k}^{({SFN})}w_{b,k}^{({SFN})}} = \left\lbrack {\left( {\sum\limits_{\overset{\_}{b}}\; h_{\overset{\_}{b},k}} \right)w_{\overset{\_}{b},k}} \right\rbrack},{\overset{\_}{b} = b},b^{\prime},b^{''}}\end{matrix} \right.} & (2)\end{matrix}$

where h_(b,k) represents an 1×N_(t) channel vector between a home basestation b and a terminal k, w_(b,k) represents an N_(t)×1 transmitweight vector, s_(b,k) represents a transmit symbol, h_(b,k) representsa 1×N_(t) interference channel vector between a neighbor base station b′and a terminal k, w_(b′,1) and s_(b′,1) are an N_(t)×1 transmit weightvector of a neighbor base station b′ and a transmit symbol,respectively, S_(b) represents a set of terminals serviced by a basestation b, and B represents a set of all base stations.

According to one embodiment of the present invention a base station mayefficiently determines a precoding scheme according to a receptionsignals from terminals and service the terminals classified according tothe determined precoding scheme.

In the above-described precoding scheme, a local precoding scheme isalso called a non-coherent precoding scheme and generally refers to ascheme for providing the same data service to one or multiple terminalsusing a precoding weight different for each cell based on CDIinformation for each cell.

As in Equation (2), three base stations are configured with a precodingweight w _(b,k) independently with respect to a channel h_(b,k),b=b,b′,b″, respectively, to a k-th terminal and use the same tocommunicate with the terminals.

Since a reception end performs non-coherent combining for an independentchannel, it may not obtain an optimized performance. In addition, sincethe reception end should feed back CDI information regarding eachindividual channel, an overhead for feedback may be relatively large.

The SFN precoding scheme is also called a coherent precoding scheme, andgenerally refers to a transmission scheme in which multiple cellsprovide the same data service to one or multiple terminals using thesame precoding weight.

As in Equation (2), a form where channels h _(b,k), b=b,b′,b″ from threebase stations to an k-th terminal are summed may be expressed by

$\left( {\sum\limits_{\overset{\_}{b}}\; h_{\overset{\_}{b},k}} \right),$

and the same precoding weight w_(b,k) is configured and used.

A reception end performs coherent combining with respect to a channel toa plurality of cells. Since the SFN precoding scheme sums multiplechannels, it may be influenced to a relatively large degree by a phasedifference from each cell.

That is, when a difference exists in the phases of respective channelsto two cells, channel deterioration may occur. In other words, when twodifferent channels are summed, a phenomenon that a channel gain reducesdue to a phase difference may occur. In contrast, since the SFNprecoding scheme feeds back only CDI information regarding a summedchannel, an overhead for feedback may be relatively small.

The global precoding scheme includes a precoding scheme that extends thelocal precoding using a virtual Multiple Input Multiple Output (MIMO)channel, and refers to a precoding scheme for performing transmissionvia multiple channels formed from multiple cells as if it virtuallyperformed transmission via a plurality of antennas. In addition, theglobal precoding scheme refers to all transmission schemes that promiseprecoding weight like virtually transmitting multiple channels formed atmultiple transmission ends via multiple antennas such as provided byspatial multiplexing. Like the local precoding scheme, the multipletransmission ends form different precoding weights but divide one largeprecoding weight and use the divided weight by a promise between thetransmission ends. As in Equation (2), a form where channels h _(b,k),b=b,b′,b″ from three base stations to an k-th terminal are virtuallystacked may be expressed by └h_(b,k) h_(b′,k) h_(b″,k)]┘, and precodingweight w _(b,k) are promised and used, respectively, to virtually formone large precoding weight └w_(b,k) w_(b′,k) w_(b″,k)┘. Therefore, areception end may expect a spatial multiplexing effect obtained bymultiple virtually formed channels. However, since CDI informationregarding respective individual channels for virtual channelconfiguration should be fed back, an overhead for feedback may berelatively large.

Generally, for a multiple antenna transmission system to apply a beamforming technique, a terminal that receives the beam formed by the beamforming technique should feed back channel information of the terminal.In case of assuming a limited feedback environment, a codebook forquantizing channel information may be defined. A codebook characteristicaccording to each precoding scheme on the assumption of the same numberof all feedback bits under the CoMP-JPT environment may be expressed byEquation (3).

$\begin{matrix}\left\{ \begin{matrix}{{Global}\text{:}} & {{C_{G} = \left\{ {c_{k\; 1}^{G},\ldots \mspace{14mu},c_{kN}^{G}} \right\}},{c_{kn}^{G} \in C^{1 \times {3 \cdot N_{t}}}},{n = 1},N,{N = 2^{B}}} \\{{Local}\text{:}} & {{C_{L} = \left\{ {c_{k\; 1}^{L},\ldots \mspace{14mu},c_{kN}^{L}} \right\}},{c_{kn}^{L} \in C^{1 \times N_{t}}},{n = 1},N,{N = 2^{B/2}}} \\{{SFN}\text{:}} & {{C_{S} = \left\{ {c_{k\; 1}^{S},\ldots \mspace{14mu},c_{kN}^{S}} \right\}},{c_{kn}^{S} \in C^{1 \times N_{t}}},{n = 1},N,{N = 2^{B}}}\end{matrix} \right. & (3)\end{matrix}$

Each terminal feeds back a codebook index similar to h _(b,k) that isobtained by generalizing an estimated channel of the terminal to a basestation, and uses Equation (4) as a method for selecting the index.

$\begin{matrix}{n^{({Mode})} = {\arg {\max\limits_{1 \leq j \leq N}{{{\overset{\sim}{h}}_{b,k}\left( c_{kj}^{({Mode})} \right)}^{*}}}}} & (4)\end{matrix}$

Therefore, considering the number of the same feedback bits, sinceprecoding is extended to 3N_(t) dimensions, when compared to N_(t)dimensions, a quantization error may be exhibited. Additionally, sincethe local precoding feedbacks information with independent considerationof a channel with multiple base stations, the number of quantizationbits for one channel reduces by ⅓, and so a quantization error may beexhibited. In contrast, the SFN precoding scheme may quantize a channelwithout a loss of quantization bits in N_(t) dimensions, but a channelgain may deteriorate when channels are summed due to the characteristicof the selected transmission method.

To select an efficient transmission method depending on a channel statusby reflecting this characteristic, a Signal to Interference plus NoiseRation (SINR) may be expressed by Equation (5).

$\begin{matrix}{{{SINR}_{b,k} = {\frac{I\; \rho {{h_{b,k}^{({Mode})}\left( {\hat{h}}_{b,k}^{({Mode})} \right)}^{+}}^{2}}{\begin{matrix}{1 + {\rho {h_{b,k}^{({Mode})}}^{2}{\sum\limits_{{j \in S_{b}},{j \neq k}}\; {{{\overset{\sim}{h}}_{b,k}w_{b,j}^{({Mode})}}}^{2}}} +} \\{\rho {\sum\limits_{{\hat{b} \in B},{\hat{b} \neq b},{\hat{b} \neq b^{\prime}},{\hat{b} \neq b^{''}}}\; {{h_{\hat{b},k}}^{2}{\sum\limits_{l \in S_{b^{\prime}}}\; {{{\overset{\sim}{h}}_{\hat{b},k}w_{\hat{b},l}}}^{2}}}}}\end{matrix}} = {\frac{\rho {h_{b,k}^{({Mode})}}^{2}\cos^{2}\theta_{b,k}^{({Mode})}}{1 + {\rho {h_{b,k}^{({Mode})}}^{2}\left( {\sin^{2}\theta_{b,k}^{({Mode})}} \right){\sum\limits_{{j \in S_{b}},{j \neq k}}\; {\beta \left( {1,{N_{t} - 2}} \right)}}} + {\rho \left( {ICI}_{k} \right)}} = \frac{\rho {h_{b,k}^{({Mode})}}^{2}\cos^{2}\theta_{b,k}^{({Mode})}}{E\left\lbrack \Delta^{({Mode})} \right\rbrack}}}},{1 \leq k \leq K}} & (5)\end{matrix}$

where ρ represents an SNR, (ĥ_(b,k) ^((Mode))) represents a codebookvector selected by a codebook select method, θ_(b,k) ^((Mode))represents an quantization error of a codebook according to eachprecoding scheme, β(1,N_(t)−2) represents beta distribution, andE[Δ^((Mode))] represents an expected SINR degradation according to eachprecoding scheme.

When K≦N_(t), in case of providing service to multiple terminals, inorder to predict performance deterioration by a channel of a differentterminal, an expected SINR degradation may be derived as shown inEquation (6).

$\begin{matrix}\begin{matrix}{{E\lbrack\Delta\rbrack} = {E\left\{ {1 + {\rho {h_{b,k}}^{2}\left( {\sin^{2}\theta_{b,k}} \right){\sum\limits_{j \neq k}\; {\beta \left( {1,{N_{t} - 2}} \right)}}} + {\rho \left( {ICI}_{k} \right)}} \right\}}} \\{= {1 + {\rho \; E\left\{ {h_{b,k}}^{2} \right\} E\left\{ {\sin^{2}\theta_{b,k}} \right\} \left( {N_{t} - 1} \right)E\left\{ {\beta \left( {1,{N_{t} - 2}} \right)} \right\}} +}} \\{{\rho \; E\left\{ {ICI}_{k} \right\}}} \\{= {1 + {\rho \; {N_{t}\left( {\int_{0}^{1}{x\ {{F_{\sin^{2}\theta}(x)}}}} \right)}\left( {N_{t} - 1} \right)\frac{1}{N_{t} - 1}} + {\rho \; E\left\{ {ICI}_{k} \right\}}}} \\{= {1 + {{P\left( \frac{N_{t} - 1}{N_{t}} \right)}2^{\frac{B}{N_{t} - 1}}} + C}}\end{matrix} & (6)\end{matrix}$

In the expected SINR degradation,

$2^{\frac{B}{N_{t} - 1}}$

represents an upper bound of a quantization error by Voroni region, andis determined by the number of transmission antennas that generate achannel and the number of quantization bits. Additionally,

$\frac{N_{t} - 1}{N_{t}}$

represents a factor reflecting an interference between terminals by aninfluence of a quantization error, and is determined by a rank of aservice channel determined by the number of serviced terminals.Therefore, the expected SINR degradation may be reduced to Equation (7).

$\begin{matrix}{{E\lbrack\Delta\rbrack} = {1 + {{P\left( \frac{R - 1}{R} \right)}2^{\frac{B}{N_{t} - 1}}} + C}} & (7)\end{matrix}$

where R represents a rank of a service channel. That is, the derivedexpected SINR degradation is determined by transmit power P of atransmission end, the number R of ranks of service channels between atransmission end and a reception end, the number B of feedback bits of acodebook used in the system, and the number N_(t) of antennas of thetransmission end. Physically, the expected SINR degradation denotes aninterference to a multiple link between a transmission end and areception end.

The number of interferences of other links for one link in an MIMOchannel between an actual transmission end and an actual reception endhas an influence having the form of

$\left( \frac{R - 1}{R} \right),$

and has interference strength with transmission power P of atransmission end.

In a codebook formed via a Quantization cell Upper Bound (QUB) havinguniform distribution, a space of a codebook may be formed by the numberB of feedback bits, and the number N_(t) of antennas of a transmissionend. In this case, when a quantization error that may occur is definedas an upper bound, it can be expressed by

$2^{- \frac{B}{N_{t} - 1}}.$

That is, since an increase of the number B of feedback bits causesreduction of the upper bound of a quantization error, generation of asmaller quantization error may be reflected.

In contrast, since an increase of the number N_(t) of antennas of atransmission end means that a codebook space extends to multipledimensions, it may cause an increase of the upper bound of thequantization error and so generation of a greater quantization error isreflected.

An interference C from a cell that does not perform cooperation is anarbitrary constant and is not a factor that has an influence in theproposed scheme and therefore, may be designated arbitrarily. Theexpected SINR degradation is reduced according to the above-derivedprecoding scheme and expressed as in Equation (8).

$\begin{matrix}{E\left\lbrack {\Delta^{({Mode})}\left\{ \begin{matrix}{{{Global}\text{:}\mspace{14mu} {E\left\lbrack \Delta^{({Global})} \right\rbrack}} = {1 + {P\left\lbrack {{\frac{\left( {R^{({Global})} - 1} \right)}{\left. R^{({Global})} \right)}2^{{- B}/{({{3 \cdot N_{t}} - 1})}}} + C} \right.}}} \\{{{Local}\text{:}\mspace{14mu} {E\left\lbrack \Delta^{({Local})} \right\rbrack}} = {1 + {P\left\lbrack {{\frac{\left( {R^{({Local})} - 1} \right)}{\left. R^{({Local})} \right)}2^{- {({{B/3}/{({N_{t} - 1})}}}}} + C} \right.}}} \\{{{SFN}\text{:}\mspace{20mu} {E\left\lbrack \Delta^{({SFN})} \right\rbrack}} = {1 + {P\left\lbrack {{\frac{\left( {R^{({SFN})} - 1} \right)}{\left. R^{({SFN})} \right)}2^{{- B}/{({N_{t} - 1})}}} + C} \right.}}}\end{matrix} \right.} \right.} & (8)\end{matrix}$

where R represents a rank of a service channel, P representstransmission power of a transmission end, and B represents the number Bof feedback bits of a codebook used in the system. That is, respectiveprecoding schemes reflect the characteristic of a relevant precodingscheme on the assumption of the same number B of feedback bits, so thatan interference for a multiple link between an actual transmission endand an actual reception end has a different form. Therefore, a terminalmay determine a CDI and a CQI depending on a precoding scheme withconsideration of expected SINR degradation.

FIG. 1 illustrates an example mobile communication system according toan embodiment of the present invention.

Referring to FIG. 1, the mobile communication system includes aplurality of base stations 100, 110, and 120, and a plurality ofterminals 150, 160, and 170. Each base station shares channelinformation of a terminal to service via a backhaul network. Althoughthe base stations 100, 110, and 120 are similar in design andconstruction, only one base station 100 is described herein forsimplicity of description. The base station 100 includes a ZF beamformer 108, a scheduler 105, and a user grouping unit 106. The scheduler105 includes a mode selector 104, a global Semi-orthogonal UserScheduling (SUS) unit 101, a local SUS unit 102, and an SFNSUS unit 103.In addition, although the terminals 150, 160, and 170 are similar indesign and construction, only one terminal 150 is described herein. Theterminal 150 includes a mode selector 154, a global CQI unit 151, alocal CQI unit 152, and an SFNCQI unit 153. In the drawing, other knownfunctional blocks required for general operations of the base stationand the terminal have been omitted for ease of description.

The global CQI unit 151, the local CQI unit 152, and the SFNCQI unit 153receive a training sequence (a reference signal) from respective basestations 100, 110, 120 that perform cooperative transmission to estimatechannel information and determine a CQI. At this point, thedetermination of the CQI is performed with consideration of expectedSINR degradation. Here, since the terminals 150, 160, and 170 cannotestimate an accurate rank of a serviced channel when determiningexpected SINR degradation, the terminals assume that the channel has amaximum rank. In the present invention, the global CQI unit 151, thelocal CQI unit 152, and the SFNCQI unit 153 may be referred to as areceiver. The receiver may receive data precoded according to aprecoding scheme determined by the base stations 100, 110, and 120.

The mode selector 154 selects a precoding scheme maximizing a CQI andfeeds back a determined CQI and CDI to a base station. Here, CQIinformation is transmitted to a home base station is requesting ahandover, CDI information may be distributed and transmitted to the homebase station and other base stations depending on a transmission method,and a feedback amount of the CDI information is fed back with fixed bitsof B bits in all transmission methods. Though not shown in the drawings,the terminal 150 may feed back a CQI, a CDI, and information regarding aprecoding scheme to one base station 100, 110, or 120.

The user grouping unit 106 collects fed-back CQI and CDI of a terminalto perform user grouping according to a precoding scheme. That is, theuser grouping unit 106 collects terminals that use the same precodingscheme to set respective initial candidate group sets.

After that, the global SUS unit 101, the local SUS unit 102, and theSFNSUS unit 103 perform individual user scheduling on each precodingscheme to generate a service candidate group set.

The mode selector 104 selects a precoding scheme that maximizes atransmission capacity of a service candidate group set with respect toeach precoding scheme, and shares this precoding scheme with cooperatingneighbor base stations via a backhaul network (Joint transmission). Inaddition, the ZF beam fainter 108 transmits a symbol (data) to aselected service candidate group set via a transmitter that may includea modem. The transmitter may transmit a training sequence (a referencesignal) to a terminal.

FIG. 2 illustrating an example message flow according to an embodimentof the present invention.

Referring to FIG. 2, a terminal 210 that receives a service of aCoMP-JPT receives a training sequence (a reference signal) from eachbase station performing cooperative transmission (step A) to estimatechannel information (step B).

The terminal 210 selects a CDI by each precoding scheme based on theestimated channel information, and determines a corresponding CQI usingEquation (9) (step C). At this point, the determination of the CQI isperformed with consideration of expected SINR degradation. Here, sincethe terminal 210 cannot estimate an accurate rank of a serviced channelwhen determining expected SINR degradation, the terminal assumes thatthe channel has a maximum rank (R_(G)=3V, R_(L)=R_(S)=V)

$\begin{matrix}{{{CQI}\left( h_{b,k}^{({Mode})} \right)} = {{SINR}_{b,k}^{({Mode})} = \frac{\rho {h_{b,k}^{({Mode})}}^{2}\cos^{2}\theta_{b,k}^{({Mode})}}{E\left\lbrack \Delta^{({Mode})} \right\rbrack}}} & (9)\end{matrix}$

where ρ represents an SNR, h_(b,k) ^((Mode)) represents a channel vectoron a base station b and a user k according to a precoding scheme (mode),θ_(b,k) ^((Mode)) represents a quantization error of a codebookaccording to a precoding scheme, and E[Δ^((Mode))] represents expectedSINR degradation according to each precoding scheme.

After that, a terminal selects a precoding scheme that maximizes a CQIusing Equation (10) (step D) and feeds back a determined CQI and adetermined CDI to a base station (step E). Here, CQI information istransmitted to a home cell that is requesting a handover, CDIinformation may be distributed and transmitted to the home cell and abase station depending on a transmission method, and a feedback amountof the CDI information is fed back with fixed bits of B bits in alltransmission methods according to Equation (10):

$\begin{matrix}\left\{ \begin{matrix}{{{CQI}\text{:}\mspace{14mu} {CQI}^{({Mode})}} = {\max\limits_{Mode}{{CQI}\left( h_{b,k}^{({Mode})} \right)}}} \\{{{{CDI}\text{:}\mspace{14mu} n^{({Mode})}} = {\arg \; {\max\limits_{1 \leq j \leq N}\underset{1 \leq j \leq N}{{{\overset{\sim}{h}}_{b,k}\left( c_{kj}^{({Mode})} \right)}^{*}}}}},{c_{kj} \in C_{Mode}}}\end{matrix} \right. & (10)\end{matrix}$

where, h_(b,k) ^((Mode)) represents a channel vector on a base station band a user k according to a precoding scheme (mode), and c represents acodebook.

After that, a base station that services a terminal collects fed-backinformation of a terminal (step F), and performs terminal grouping (oruser grouping) according to a precoding scheme using Equation (11) (stepG). That is, the base station collects terminals that use the sameprecoding scheme to set respective initial candidate group sets.

$\begin{matrix}{{M_{0}^{({Mode})} = \left\{ {{Mode}_{1},\ldots \mspace{14mu},{Mode}_{k}} \right\}},{{\sum\limits_{Mode}{A_{0}^{({Mode})}}} = K}} & (11)\end{matrix}$

where |A₀ ^((Mode))| represents a size of an initial candidate group setof each precoding scheme, and the sum of sizes of respective initialcandidate group sets is equal to the number of all cooperativetransmission candidate terminals. In addition, the term Mode representseach precoding scheme. In addition, K represents the number of alltransmission candidate terminals.

After that, a base station performs terminal grouping via aZFBF-Semi-orthogonal User Scheduling algorithm with consideration ofProportional Fairness (PF) with respect to a precoding scheme for eachinitial candidate group set (step H). The base station performsindividual user scheduling according to each precoding scheme togenerate a service candidate group set of A_(i) ^((Mode)) whereterminals up to an i-th terminal have been selected.

After that, the base station selects a precoding scheme maximizing atransmission capacity of a service candidate group set of A_(i)^((Mode)) according to Equation (12) with respect to each precodingscheme (step I), shares this precoding scheme with cooperating neighborbase stations via a backhaul network (Joint Transmission) (step J), andtransmits a symbol to a selected service candidate group set (step K).

Since the base station may accurately know an accurate rank of aserviced channel via the size of a service candidate group set, the basestation determines expected SINR degradation using R_((Mode))=|A_(i)^((Mode))|.

$\begin{matrix}{{{{Selection}\mspace{14mu} {Criteria}\text{:}\mspace{14mu} {\max\limits_{Mode}{\sum\limits_{k \in A_{i}^{({Mode})}}{{\mu_{b,k}(t)} \cdot {\log_{2}\left( {1 + {SINR}_{b,k}^{({Mode})}} \right)}}}}},{where}}{{SINR}_{b,k}^{({Mode})} = \frac{\rho {{CQI}^{({Mode})}}^{2}}{E\left\lbrack \Delta^{({Mode})} \right\rbrack}}} & (12)\end{matrix}$

where the term Mode represents each precoding scheme, CQI (Mode)represents a CQI according to a precoding scheme, E[Δ^((Mode))]represents expected SINR degradation and so is an interference to amultiple link between a transmission end and a reception end accordingto a precoding scheme, and μ_(b,k)(t) represents a utility function forreflecting Proportional Fairness (PF).

As described, the present invention derives expected SINR degradationwith consideration of a characteristic of a CoMP-JPT precodingtechnology, and utilizes the same to provide communication with betweenthe base stations and one or more terminals. Derivation and utilizationof expected SINR degradation may be utilized by a transmission end of alimited feedback environment in determining an effective transmissionmethod by estimating an inaccuracy degree of a codebook vector.

FIG. 3 illustrates an example process for operating a terminal accordingto an embodiment of the present invention.

Referring to FIG. 3, when receiving a training sequence (a referencesignal) from a base station at step 310, a terminal performs channelestimation on the reference signal at step 320.

After that, the terminal selects a CDI according to each precodingscheme based on the estimated channel information, and determines acorresponding CQI at step 330. At this point, the determination of theCQI is performed with consideration of expected SINR degradation. Sincethe terminal cannot estimate an accurate rank of a serviced channel whendetermining the expected SINR degradation, the terminal may assume thatthe channel has a maximum rank.

The terminal selects a precoding scheme maximizing a CQI, and feeds backdetermined CQI and CDI to a base station at step 340. At this point,information regarding a determined precoding scheme may be included.

The terminal receives data from the base station according to thedetermined precoding scheme at step 350. After this, the process asshown in FIG. 3 has ended.

FIG. 4 illustrates an example process for operating a base stationaccording to an embodiment of the present invention.

Referring to FIG. 4, the base station periodically transmits a trainingsequence (a reference signal) to a terminal at step 410.

When receiving a CDI and a CQI from a terminal at step 420, the basestation collects the fed-back information (CDI and CQI) of the terminalto perform terminal grouping (or user grouping) according to eachprecoding scheme at step 430. That is, the base station collectsinformation associated with the terminals that use the same precodingscheme to set respective initial candidate group sets. Informationregarding a determined precoding scheme may be included in theinformation of the terminal.

The base station performs individual user scheduling according to eachprecoding scheme to generate a service candidate group set at step 440.

The base station determines a precoding scheme that maximizes atransmission capacity of a service candidate group set with respect toeach precoding scheme, and shares this precoding scheme with neighborbase stations via a backhaul at step 450.

The base station transmits a symbol (data) to a determined candidategroup set using the determined precoding scheme at step 460. After this,the process of FIG. 4 has ended.

FIG. 5A and FIG. 5B illustrates example performance analysis results ofa system according to an embodiment of the present invention.

Referring to FIG. 5A and FIG. 5B in a state where each sector of a cellis divided into an inner region that does not perform CoMP and an outerregion that does perform CoMP, three contiguous sectors are set toperform CoMP on one another such that an influence of an interferencebetween them may be reduced, the performance of the system is analyzed.

The system proposed by the present invention is segregated into aGlobal, Local, SFN Selection (GLS) mode, and a Global, SFN Selection(GS) mode, and determines a precoding scheme to operate using modedetermination. Here, the GLS is a scheme considering all of respectiveprecoding technologies, and GS is a scheme that excludes a localprecoding. FIG. 5A is an average sum rate of a service terminal and FIG.5B is a sum rate of a CoMP terminal corresponding to an outer 5% of eachcell region.

The proposed system shows a better performance result compared to areference system on the whole. When compared to an SFN system and aglobal precoding system having best performance among the referencesystems, the proposed system is expected to have transmission capacityperformance improvement of 3% to 33% depending on a threshold.

In addition, when compared to an SFN system and a global precodingsystem having best performance among the reference systems, aperformance corresponding to an outer 5% is expected to havetransmission capacity performance improvement of 3% to 22% depending ona threshold.

Therefore, under a limited feedback environment, a gain of a modeselection technology that may efficiently reflect an influence of aquantization error may be obtained.

The present invention has an advantage of efficiently servicing multipleusers with consideration of a time-varying channel environment. Sincethe precoding scheme according, to the present invention exhibits adifference in a transmission capacity depending on a channelenvironment, an efficient precoding scheme may be selected, andaccordingly, a transmission capacity of an entire system may beincreased. More particularly, certain embodiments of the presentinvention may have an advantage of obtaining a higher transmissioncapacity by selecting a more efficient transmission method within afeedback range of fixed B bits under a limited feedback environment.

Since expected SINR degradation proposed by the present invention hasbeen derived by reflecting, the characteristics of respective precodingtechnologies utilized in a CoMP-JPT environment, a transmission end of alimited feedback environment may determine an effective transmissionmethod by estimating an inaccuracy degree of a codebook vector.

Although the present disclosure has been described with an exemplaryembodiment, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosure encompasssuch changes and modifications as fall within the scope of the appendedclaims.

1. A method for selecting a precoding mode in a mobile communicationsystem by a base station, the method comprising: when receiving channelestimate information from at least one terminal, collecting the channelestimate information to perform user grouping according to a precodingscheme from among a plurality of precoding schemes; performingindividual user scheduling according to each precoding scheme togenerate a service candidate group set; determining a precoding schemethat maximizes a transmission capacity of a service candidate group setfor each precoding scheme; and transmitting data to a determinedcandidate group set using the determined precoding scheme.
 2. The methodof claim 1, further comprising sharing the channel estimate informationand the determined precoding scheme with one or more cooperatingneighbor base stations via a backhaul network.
 3. The method of claim 1,wherein the channel estimate information comprises at least one of aChannel Direction Indicator (CDI), a Channel Quality Indicator (CQI),and information regarding the determined precoding scheme.
 4. The methodof claim 1, wherein the collecting of the channel estimate informationto perform the user grouping according to the precoding scheme furthercomprises: collecting terminals that use the same precoding scheme toset respective initial candidate group sets according to the followingEquation:${M_{0}^{({Mode})} = \left\{ {{Mode}_{1},\ldots \mspace{14mu},{Mode}_{k}} \right\}},{{\sum\limits_{Mode}{A_{0}^{({Mode})}}} = K}$where |A₀ ^((Mode))| represents a size of an initial candidate group setof each precoding scheme, sum of sizes of the respective initialcandidate group sets is equal to the number of all cooperativetransmission candidate terminals, Mode represents each precoding scheme,and K represents the number of all transmission candidate terminals. 5.The method of claim 1, wherein the performing of the individual userscheduling according to each precoding scheme to generate the servicecandidate group set further comprises: performing individual userscheduling according to the precoding scheme to generate a servicecandidate group set where users up to a specified number of users havebeen selected.
 6. The method of claim 1, wherein the determining of theprecoding scheme that maximizes the transmission capacity of the servicecandidate group set for each precoding scheme further comprises:determining the precoding scheme using according to the followingEquation:${{Selection}\mspace{14mu} {Criteria}\text{:}\mspace{14mu} {\max\limits_{Mode}{\sum\limits_{k \in A_{i}^{({Mode})}}{{\mu_{b,k}(t)} \cdot {\log_{2}\left( {1 + {SINR}_{b,k}^{({Mode})}} \right)}}}}},{where}$${SINR}_{b,k}^{({Mode})} = \frac{\rho {{CQI}^{({Mode})}}^{2}}{E\left\lbrack \Delta^{({Mode})} \right\rbrack}$where Mode represents each precoding scheme, CQI (Mode) represents a CQIaccording to the precoding scheme, E[Δ^((Mode))] represents an expectedSINR degradation such that the expected SINR represents an interferenceto a multiple link between a transmission end and a reception endaccording to the precoding scheme, and μ_(b,k)(t) represents a utilityfunction for reflecting Proportional Fairness (PF).
 7. A method forselecting a precoding mode in a mobile communication system by aterminal, the method comprising: performing channel estimation on areference signal received from a base station to select a precodingscheme from among a plurality of precoding schemes; feeding back channelinformation regarding the selected precoding scheme to the base station;and receiving data from the base station according to the selectedprecoding scheme.
 8. The method of claim 7, wherein the performing ofthe channel estimation comprises: selecting a Channel DirectionIndicator (CDI) according to each precoding scheme and determining acorresponding Channel Quality Indicator (CQI) according to the followingEquation:${{CQI}\left( h_{b,k}^{({Mode})} \right)} = {{SINR}_{b,k}^{({Mode})} = \frac{\rho {h_{b,k}^{({Mode})}}^{2}\cos^{2}\theta_{b,k}^{({Mode})}}{E\left\lbrack \Delta^{({Mode})} \right\rbrack}}$where ρ represents an SNR, h_(b,k) ^((Mode)) represents a channel vectoron a base station b and a user k according to a precoding scheme (mode),θ_(b,k) ^((Mode)) represents a quantization error of a codebookaccording to the selected precoding scheme, and E[Δ^((Mode))] representsan expected SINR degradation according to the selected precoding scheme.9. The method of claim 7, wherein the selecting of the precoding schemecomprises: selecting a precoding scheme maximizing a CQI according tothe following Equation: $\quad\left\{ \begin{matrix}{{{CQI}\text{:}\mspace{14mu} {CQI}^{({Mode})}} = {\max\limits_{Mode}{{CQI}\left( h_{b,k}^{({Mode})} \right)}}} \\{{{{CDI}\text{:}\mspace{14mu} n^{({Mode})}} = {\arg {\max\limits_{1 \leq j \leq N}\underset{1 \leq j \leq N}{{{\overset{\sim}{h}}_{b,k}\left( c_{kj}^{({Mode})} \right)}^{*}}}}},{c_{kj} \in C_{Mode}}}\end{matrix} \right.$ where, h_(b,k) ^((Mode)) represents a channelvector on a base station b and a user k according to the selectedprecoding scheme (mode), and c represents a codebook.
 10. The method ofclaim 7, wherein the channel information regarding the determinedprecoding scheme comprises at least one of a Channel Direction Indicator(CDI), a Channel Quality Indicator (CQI), and information regarding theselected precoding scheme.
 11. A base station configured to select aprecoding mode in a mobile communication system, the base stationcomprising: a user grouping unit configured to, when receiving channelestimate information from at least one terminal, collect the channelestimate information to perform user grouping according to a precodingscheme from among a plurality of precoding schemes; a schedulerconfigured to perform individual user scheduling according to eachprecoding scheme to generate a service candidate group set, anddetermine a precoding scheme that maximizes a transmission capacity of aservice candidate group set for each precoding scheme; and a transmitterconfigured to transmit data to a determined candidate group set usingthe determined precoding scheme.
 12. The apparatus of claim 11, whereinthe scheduler shares the channel estimate information and the determinedprecoding scheme with cooperating neighbor base stations via a backhaulnetwork.
 13. The apparatus of claim 11, wherein the channel estimateinformation comprises at least one of a Channel Direction Indicator(CDI), a Channel Quality Indicator (CQI), and information regarding thedetermined precoding scheme.
 14. The apparatus of claim 11, wherein whencollecting the channel estimate information to perform the user groupingaccording to the precoding scheme, the user grouping unit is configuredto collect terminals that use the same precoding scheme to setrespective initial candidate group sets according to the followingEquation:${M_{0}^{({Mode})} = \left\{ {{Mode}_{1},\ldots \mspace{14mu},{Mode}_{k}} \right\}},{{\sum\limits_{Mode}{A_{0}^{({Mode})}}} = K}$where |A₀ ^((Mode))| represents a size of an initial candidate group setof each precoding scheme, sum of sizes of respective initial candidategroup sets is equal to the number of all cooperative transmissioncandidate terminals, Mode represents each precoding scheme, and Krepresents the number of all transmission candidate terminals.
 15. Theapparatus of claim 11, wherein when performing the individual userscheduling according to each precoding scheme to generate the servicecandidate group set, the scheduler is configured to perform individualuser scheduling according to the determined precoding scheme to generatethe service candidate group set where users up to a specified number ofusers have been selected.
 16. The apparatus of claim 11, wherein whendetermining the precoding scheme maximizing the transmission capacity ofthe service candidate group set for each precoding scheme, the scheduleris configured to determine the precoding scheme according to thefollowing Equation:${{Selection}\mspace{14mu} {Criteria}\text{:}\mspace{14mu} {\max\limits_{Mode}{\sum\limits_{k \in A_{i}^{({Mode})}}{{\mu_{b,k}(t)} \cdot {\log_{2}\left( {1 + {SINR}_{b,k}^{({Mode})}} \right)}}}}},{where}$${SINR}_{b,k}^{({Mode})} = \frac{\rho {{CQI}^{({Mode})}}^{2}}{E\left\lbrack \Delta^{({Mode})} \right\rbrack}$where Mode represents each precoding scheme, CQI (Mode) represents a CQIaccording to the determined precoding scheme, E[Δ^((Mode))] representsan expected SINR degradation such that the SINR degradation comprises aninterference to a multiple link between a transmission end and areception end according to the determined precoding scheme, andμ_(b,k)(t) represents a utility function configured to reflect aProportional Fairness (PF).
 17. A terminal configured to select aprecoding mode in a mobile communication system, the terminalcomprising: a receiver configured to perform channel estimation on areference signal received from a base station, and receive data from thebase station according to a determined precoding scheme; a mode selectorconfigured to select a precoding scheme according to a channel estimateresult; and a transmitter configured to feed back channel informationregarding the determined precoding scheme to the base station.
 18. Theapparatus of claim 17, wherein when performing the channel estimation,the receiver is configured to select a Channel Direction Indicator (CDI)according to each precoding scheme and determine a corresponding ChannelQuality Indicator (CQI) according to the following Equation:${{CQI}\left( h_{b,k}^{({Mode})} \right)} = {{SINR}_{b,k}^{({Mode})} = \frac{\rho {h_{b,k}^{({Mode})}}^{2}\cos^{2}\theta_{b,k}^{({Mode})}}{E\left\lbrack \Delta^{({Mode})} \right\rbrack}}$where ρ represents an SNR, h_(b,k) ^((Mode)) represents a channel vectoron a base station b and a user k according to the determined precodingscheme (mode), θ_(b,k) ^((Mode)) represents a quantization error of acodebook according to the determined precoding scheme, and E[Δ^((Mode))]represents an expected SINR degradation according to the determinedprecoding scheme.
 19. The apparatus of claim 17, wherein when selectingthe precoding scheme, the mode selector is configured to select aprecoding scheme that maximizes a CQI according to the followingEquation: $\quad\left\{ \begin{matrix}{{{CQI}\text{:}\mspace{14mu} {CQI}^{({Mode})}} = {\max\limits_{Mode}{{CQI}\left( h_{b,k}^{({Mode})} \right)}}} \\{{{{CDI}\text{:}\mspace{14mu} n^{({Mode})}} = {\arg {\max\limits_{1 \leq j \leq N}\underset{1 \leq j \leq N}{{{\overset{\sim}{h}}_{b,k}\left( c_{kj}^{({Mode})} \right)}^{*}}}}},{c_{kj} \in C_{Mode}}}\end{matrix} \right.$ where, h_(b,k) ^((Mode)) represents a channelvector on a base station b and a user k according to the determinedprecoding scheme (mode), and c represents a codebook.
 20. The apparatusof claim 17, wherein the channel information regarding the determinedprecoding scheme comprises at least one of a Channel Direction Indicator(CDI), a Channel Quality Indicator (CQI), and information regarding thedetermined precoding scheme.